erff4.h

Newlib 嵌入式 C库 标准实现代码

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/* PROLOG END TAG zYx                                              */
#ifdef __SPU__
#ifndef _ERFF4_H_
#define _ERFF4_H_	1

#include <spu_intrinsics.h>

#include "expf4.h"
#include "recipf4.h"
#include "divf4.h"
#include "erf_utils.h"

/*
 * FUNCTION
 *  vector float _erff4(vector float x)
 *
 * DESCRIPTION
 *  The erff4 function computes the error function of each element of x.
 *
 *  C99 Special Cases:
 *  - erf(+0) returns +0
 *  - erf(-0) returns -0
 *  - erf(+infinite) returns +1
 *  - erf(-infinite) returns -1
 *
 */

static __inline vector float _erff4(vector float x)
{
  vector float onehalff  = spu_splats(0.5f);
  vector float zerof     = spu_splats(0.0f);
  vector float onef      = spu_splats(1.0f);
  vector float sign_mask = spu_splats(-0.0f);

  /* This is where we switch from Taylor Series to Continued Fraction approximation */
  vec_float4 approx_point = spu_splats(0.89f);

  vec_float4 xabs, xsqu, xsign;
  vec_uint4 isinf;
  vec_float4 tresult, presult, result;

  xsign = spu_and(x, sign_mask);

  /* Force Denorms to 0 */
  x = spu_add(x, zerof);

  xabs = spu_andc(x, sign_mask);
  xsqu = spu_mul(x, x);

  /*
   * Taylor Series Expansion near Zero
   */
  TAYLOR_ERFF4(xabs, xsqu, tresult);

  /*
   * Continued Fraction Approximation of Erfc().
   * erf = 1 - erfc 
   */
  CONTFRAC_ERFCF4(xabs, xsqu, presult);
  presult = spu_sub(onef, presult);

  /*
   * Select the appropriate approximation.
   */
  result = spu_sel(tresult, presult, spu_cmpgt(xabs, approx_point));

  /*
   * Special cases/errors.
   */

  /* x = +/- infinite, return +/-1 */
  isinf = spu_cmpeq((vec_uint4)xabs, 0x7F800000);
  result = spu_sel(result, onef, isinf);

  /*
   * Preserve sign in result, since erf(-x) = -erf(x)
   */
  result = spu_or(result, xsign);

  return result;
}

#endif /* _ERFF4_H_ */
#endif /* __SPU__ */